(1) Field of the Invention
The present invention relates to a line circuit (interface circuit) provided between a subscriber's telephone set and an exchange network. More particularly, it relates to a calling signal transmission apparatus connected to or arranged in the line circuit for controlling the transmission of a calling signal to a subscriber's telephone set.
(2) Description of the Related Art
Conventionally, when a call is received at a telephone set, a bell provided in the telephone set is intermittently activated by an intermittent ringing signal. The intermittent ringing signal is formed by intermittently cutting a continuous ringing signal of, for example, 20 Hz, by activation relay contacts. Thus, the intermittent ringing signal consists of a sequence in which, for example, a 20 Hz-continuous ringing signal is activated for one second followed by a no signal period of three seconds, i.e., silence, is repeated. The effective value of the amplitude of the continuous ringing signal is a high voltage of, for example, 90 V; a high energy level beyond the capability of the electronic switches constituting the communication network. When the communication network consists of a time-division digital switch, electronic circuits, and so forth, the above-mentioned continuous ringing signal, having the high effective voltage, cannot be directly transmitted on the speech path (communication line) through the communication network to the telephone set. The continuous ringing signal and the intermittent ringing signal sequence must be transmitted to the telephone set through the line after the line is cut off from the network during calling. Further, after the subscriber answers the calling signal and takes the handset off the hook, the line must be again separated from the continuous ringing signal and connected to the communication network. This control for transmitting and blocking a continuous ringing signal to and from a line is performed by control of a ringing signal transmission relay.
When a subscriber lifts the handset of the telephone in response to the ringing of the bell, that is, when the telephone set is in an off-hook state, ring trip equipment operates to restored (deenergize) the relay contacts, whereby the intermittent ringing signal is stopped. In this case, during the period from the off-hook state when the bell is ringing to the restoration of the relay contacts, that is, during the period of the ring trip operation, the 20 Hz-continuous ringing signal is sent through the communication line to the telephone set so that the signal is heard as an unpleasant tone by the subscriber. Therefore, the above-mentioned ring trip operation period should be activated as quickly as possible.
In a conventional line circuit, ring trip operation is realized, as later described in detail with reference to the drawings, mainly by hardware. That is, a direct-current loop detecting signal, produced when the telephone set is in an off-hook state during a no signal period in the intermittent ringing signal which is supplied through the relay contacts to the line circuit, is input as a silent trip detecting signal (in other words, a loop signal) through a voltage separation circuit into a control processor and is processed therein by software. However, a direct-current loop detecting signal, produced when the telephone set is in an off-hook state when a signal is activated during the intermittent ringing signal sequence, is output as a ring trip detecting signal through a voltage separating circuit different from the above to a logic circuit, and the relay contacts are then restored by the logic circuit.
When the ring trip operation, activated during a period when a signal is present, is realized only by the hardware described above, the system becomes uneconomical because the voltage separating circuits and the logic circuit must be added and thus the number of parts is increased.
Also, in a conventional system, once an off-hook state is realized while a bell is ringing, the relay contacts are fixed in the restored state by the hardware. Therefore, if the subscriber erroneously hangs-up the handset immediately after lifting it off the hook, it is impossible to reactivate (energize) the relay which is already in the restored state. Accordingly, the bell does not ring when there is an incoming call, and thus the problem arises where the subscriber cannot acknowledge the incoming call although the call is waiting for the telephone set.
Still further, there is another problem in that, if noise and the like causes an erroneous ring trip operation, i.e., an off-hook state is erroneously detected, the relay contacts are also fixed in the restored state and the bell will not ring.
The entire ring trip operation could be effected by software. However, as mentioned before, it is necessary to shorten as much as possible the time from when a subscriber lifts the handset off the hook to that when the relay contacts are restored. Therefore, if the entire ring trip operation is to be effected by the software, it must be carried out in real time. However, in this case, the software will be occupied by this operation for too long a time, in spite of the simple content of the process. This is obviously uneconomical, and thus a ring trip operation performed only by software has not been effected in practice.
From another point of view, if the ringing signal transmission relay is operated at an arbitrary timing, the high voltage of the continuous ringing signal could suddenly be impressed upon the channel or suddenly eliminated from the channel, resulting in the occurrence of impulse type noise in another channel adjoining that channel. Further, the rapid changes in current would cause electromagnetic induction, also generation noise in other circuits.
Therefore, in the past, the intermittent ringing signal was modified, for effecting a so-called dry operation of the relay contacts, by an interrupted ringing signal which is composed of, for example, a 100 millisecond nonsignal state and a successive 900 millisecond signal state, as later described in more detail with reference to the drawings. When the interrupted ringing signal is used, if the ringing signal transmission relay is operated or restored in the nonsignal state of the interrupted ringing signal, noise will not occur in the communication line during operation or restoration of the relay.
However, if the called subscriber takes the receiver off the hook in the middle of the ringing tone, the ring trip circuit operates to restore the ringing signal transmission relay. If the energy of the interrupted ringing signal at that time is high, however, the restoration of the relay generates noise on another channel for the same reasons as above.
From still another point of view, in the conventional calling signal transmission apparatus, a capacitor having a relatively large capacitance is used for detecting an off-hook state of the telephone set. This capacitor, however, causes various problems such as large external dimensions, long response time for the ring trip, or is inappropriate for message waiting and other special services, as later described in more detail with reference to the drawings.
From still another point of view, when a fault occurs in the calling signal transmission apparatus, the operation of the system itself is disordered. Therefore, the fault must be detected by some means.
Conventionally, the fault in the calling signal transmission apparatus has been detected by providing a fault detecting circuit in the calling signal transmission apparatus itself. According to this conventional fault detecting method, when a fault is detected by the above-mentioned detection circuit, the fault is informed to an external display unit or to a control processing unit.
Since the fault detecting circuit is additionally provided in the calling signal transmission apparatus and signal lines must be provided to transmit the fault to the display unit or to the central processing unit, the conventional method has disadvantages of requiring a large installation space and high cost.
Still further, when it is necessary to detect whether or not the calling signal transmission apparatus is mounted on or connected to the line circuit, a circuit for detecting the absence of mounting is necessary in addition to the fault detecting circuit, causing a further increase in the installation space and the cost.